Method and apparatus for determining the nature of subterranean reservoirs

ABSTRACT

A system for detecting a subterranean reservoir or determining the nature of a subterranean reservoir  12  whose position and geometry is known from previous seismic surveys. An electromagnetic field  24, 25, 26, 27  is applied by a transmitter  18  on the seabed  14  and detected by antenna  21, 22, 23  also on the seabed  14 . A refracted wave component is sought in the wave field response, to determine the nature of any reservoir present.

RELATED APPLICATIONS

This Application is a continuation of pending and commonly assignedinternational patent application PCT/GB01/00419, entitled “Method andApparatus for Determining the Nature of Subterranean Reservoirs”, filedFeb. 1, 2001, which claims priority to patent application GB 0002422.4filed Feb. 2, 2000, which are hereby incorporated by reference herein.

BACKGROUND

The present invention relates to a method and apparatus for determiningthe nature of submarine and subterranean reservoirs. The invention isparticularly suitable for determining, whether a reservoir, whoseapproximate geometry and location are known, contains hydrocarbons orwater, though it can also be applied to detecting reservoirs withparticular characteristics.

Currently, the most widely used techniques for geological surveying,particularly in sub-marine situations, are seismic methods. Theseseismic techniques are capable of revealing the structure of thesubterranean strata with some accuracy. However, whereas a seismicsurvey can reveal the location and shape of a potential reservoir, itcannot reveal the nature of the reservoir.

The solution therefore is to drill a borehole into the reservoir.However, the costs involved in drilling an exploration well tend to bein the region of £25 M and since the success rate is generally about 1in 10, this tends to be a very costly exercise.

SUMMARY OF THE INVENTION AND DETAILED DESCRIPTION OF PREFERREDEMBODIMENTS

It is therefore an object of the invention to provide a system fordetermining, with greater certainty, the nature of a subterraneanreservoir without the need to sink a borehole.

According to one aspect of the invention, there is provided a method ofdetermining the nature of a subterranean reservoir whose approximategeometry and location are known, which comprises: applying a timevarying electromagnetic field to the strata containing the reservoir;detecting the electromagnetic wave field response; seeking in the wavefield response, a component representing a refracted wave from thehydrocarbon layer; and determining the content of the reservoir, basedon the presence or absence of a wave component refracted by thehydrocarbon layer. According to a second aspect of the is provided amethod for searching for a containing subterranean reservoir whichinvention, there hydrocarbon comprises: applying a time varyingelectromagnetic field to subterranean strata; detecting theelectromagnetic wave field response; seeking, in the wave fieldresponse, a component representing a refracted wave; and determining thepresence and/or nature of any reservoir identified based on the presenceor absence of a wave component refracted by hydrocarbon layer.

According to a further aspect of the invention, there is providedapparatus for determining the nature of a subterranean reservoir whoseapproximate geometry and location are known, or for searching for ahydrocarbon containing subterranean reservoir, the apparatus comprising:means for applying a time varying electromagnetic field to the stratacontaining the reservoir; means for detecting the electromagnetic wavefield response; and means for seeking, in the wave field response, acomponent representing a refracted wave, thereby enabling the presenceand/or nature of a reservoir to be determined.

It has been appreciated by the present applicants that while the seismicproperties of oil-filled strata and water-filled strata do not differsignificantly, their electromagnetic resistivities and permittivitiesdiffer. Thus, by using an electromagnetic surveying method, thesedifferences can be exploited and the success rate in predicting thenature of a reservoir can be increased significantly. This representspotentially an enormous cost saving.

The present invention arises from an appreciation of the fact that whenan (electromagnetic) EM field is applied to subterranean strata whichinclude a reservoir, in addition to a direct wave component and areflected wave component from the reservoir, the detected wave fieldresponse will include a “refracted” wave component from the reservoir.The reservoir containing hydrocarbon is acting in some way as a waveguide. For the purposes of this specification, however, the wave will bereferred to as a “refracted wave”, regardless of the particularmechanism which in fact pertains.

Be that as it may, a refracted wave behaves differently, depending onthe nature of the stratum in which it is propagated. In particular, thepropagation losses in hydrocarbon stratum are much lower than in awater-bearing stratum while the speed of propagation is much higher.Thus, when an oil-bearing reservoir is present, and an EM field isapplied, a strong and rapidly propagated refracted wave can be detected.This may therefore indicate the presence of the reservoir or its natureif its presence is already known. Preferably, therefore, the methodaccording to the invention further includes the step of analyzing theeffects on any detected refracted wave component that have been causedby the reservoir in order to determine further the content of thereservoir, based on the analysis.

Preferably, the applied electromagnetic field is polarized. Preferably,the polarization is such as if created by in-line horizontal transmitterand receiver antennas.

If the offset between the transmitter and receiver is significantlygreater than three times the depth of the reservoir from the seabed(i.e., the thickness of the overburden), it will be appreciated that theattenuation of the refracted wave will often be less than that of directwave and the reflected wave. The reason for this is the fact that thepath of the refracted wave will be effectively distance from thetransmitter down to the reservoir i.e., the thickness of the overburden,plus the offset along the reservoir, plus the distance from thereservoir up to the receivers i.e., once again the thickness of theoverburden.

The polarization of the source transmission will determine how muchenergy is transmitted into the oil-bearing layer in the direction of thereceiver. A dipole antenna is therefore the preferred transmitter,though any transmitter capable of generating an appropriate polarizedfield can be used. In general, it is preferable to adopt a dipole with alarge effective length. The transmitter dipole may therefore be 100 to1000 meters in length, and may be 10 to 1000 meters preferablycross-polarized. The receiver Dipole optimum length is determined by thethickness of the overburden.

The technique is applicable in exploring land-based subterraneanreservoirs but is especially applicable to submarine, in particularsub-sea, subterranean reservoirs. Preferably the field is applied usingone or more transmitters located on the earth's surface, and thedetection is carried out by one or more receivers located on the earth'ssurface. In a preferred application, the transmitter(s) and/or receiversare located on or close to the seabed or the bed of some other area ofwater. Conveniently, there will be a single transmitter and an array ofreceivers, the transmitter(s) and receivers being dipole antennae orcoils, though other forms of transmitter/receivers can be used. Thetransmitter may be in an existing well. Also, if improved directionalityof the emitted field is desirable, then a plurality of transmitters withphase adjustment can be used.

In one arrangement, a single transmitter and several receivers arearranged on a single cable which is laid in the required position on theseabed by a surface or submarine vessel. These can then be moved toanother location. In a second arrangement, several receivers have fixedpositions on the seabed. The transmitter can be moved to differentlocations. In a third arrangement, a transmitter may be positioned by afirst vessel while a second vessel positions one or more receivers. Thisaffords flexibility in the positioning of both transmitter andreceivers. In a fourth arrangement, that the transmitter be in anexisting well while the receivers may constitute a fixed matrix or theymay be movable.

It will be appreciated that the present invention may be used todetermine the position, the extent, the nature and the volume of aparticular stratum, and may also be used to detect changes in theseparameters over a period of time. Electromagnetic surveying techniquesin themselves are known. However, they are not widely used in practice.In general, the reservoirs of interest are about 1 km or more below theseabed. In order to carry out electromagnetic surveying as a stand alonetechnique in these conditions, with any reasonable degree of resolution,short wavelengths are necessary. Unfortunately, such short wavelengthssuffer from very high attenuation. Long wavelengths do not provideadequate resolution. For these reasons, seismic techniques arepreferred.

However, while longer wavelengths applied by electromagnetic techniquescannot provide sufficient information to provide an accurate indicationof the boundaries of the various strata, if the geological structure isalready known, they can be used to determine the nature of a particularidentified formation, if the possibilities for the nature of thatformation have significantly differing electromagnetic characteristics.The resolution is not particularly important and so longer wavelengthswhich do not suffer from excessive attenuation can be employed.

The resistivity of seawater is about 0.3 ohm-m and that of theoverburden beneath the seabed would typically be from 0.3 to 4 ohm-m,for example about 2 ohm-m. However, the resisitivty of an oil reservoiris likely to be about 20-300 ohm-m. This large difference can beexploited using the techniques of the present invention. Typically, theresisitvity of a hydrocarbon-bearing formation will be 20 to 300 timesgreater than water-bearing formation.

Due to the different electromagnetic properties of a gas/oil bearingformation and a water bearing formation, one can expect a reflection andrefraction of the transmitted field at the boundary of a gas/oil bearingformation. However, the similarity between the properties of theoverburden and a reservoir containing water means that no reflection orrefraction is likely to occur.

The transmitted field may be pulsed, however, a coherent continuous wavewith stepped frequencies is preferred. It may be transmitted for asignificant period of time, during which the transmitter shouldpreferably be stationary (although it could be moving slowly), and thetransmission stable. Thus, the field may be transmitted for a period oftime from 3 seconds to 60 minutes, preferably from 3 to 30 minutes, forexample about 20 minutes. The receivers may also be arranged to detect adirect wave and a wave refracted from the reservoir, and the analysismay include extracting phase and amplitude data of the refracted wavefrom corresponding data from the direct wave.

Preferably, the wavelength of the transmission is given by the formula0.1S≦λ≦5S;where λ is the wavelength of the transmission through the overburden andS is the distance from the seabed to the reservoir. More preferably λ isfrom about 0.5S to 2S. The transmission frequency may be from 0.01 Hz to1 kHz, preferably from 1 to 20 Hz, for example 5 Hz.

In a preferred regime, a first transmission is made at a first frequencyand received by each receiver in a tuned array of receivers, then asecond transmission is made at a second frequency and received by thesame tuned array of receivers, the receivers being tuned to receivetheir respective transmission. This would probably be repeated severalmore times, though it may only be carried out once.

Preferably, the analysis includes comparing the results of themeasurements taken with the results of a mathematical simulation modelbased on the known properties of the reservoir and overburdenconditions.

Preferably, the distance between the transmitter and a receiver is givenby the formula0.5λ≦L≦10λ;where λ is the wavelength of the transmission through the overburden andL is the distance between the transmitter and the first receiver.

Given that the distances and the geometry of the reservoir will be knownfrom previous seismic surveys, an optimum X and L would be selected.

Preferably, the analyzing means is arranged to analyze phase andamplitude. The data can be analyzed using time domain and frequencydomain techniques, and other pulse sharpening techniques. Thus, the datacan be made to mimic seismic data so that conventional seismicpost-processing techniques can be employed.

If a location of interest is considered, a mathematical modelingoperation may be carried out. Thus, the various relevant parameters,such as depth and expected resistivities of the various known strata inthe overburden are applied to the mathematical model and the expectedresults are calculated in dependence upon whether a formation underconsideration is oil-bearing or water-bearing. The theoreticallypredicted results can then be compared with the actual results achievedin the field in order to determine the nature of the formation.

The present invention also extends to a method of surveying subterraneanmeasures which comprises; performing a seismic survey to determine thegeological structure of a region; and where that survey reveals thepresence of a subterranean reservoir, subsequently performing a methodas described above.

The invention may be carried into practice in various ways and someembodiments will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic diagram of an experimental technique verifying theprinciples of the invention.

FIG. 2 is a schematic section of a system in accordance with theinvention.

FIG. 1 shows a test rig comprising a tank 11 filled with seawater and asimulated oil-bearing layer, in the form of a diaphragm 12 filled withfresh water. The diaphragm 12 is suspended above the bottom of the tank11. A transmitter 13 and a receiver 14 are mounted on respectivevertical posts 15, 16 suspended from a beam 17. The posts are at aconstant spacing L and the transmitter 13 and receiver 14 are verticallymovable up and down their posts 15, 16.

When the transmitter 13 and receiver 14 are in the position shown insolid lines, the sensitivity of the receiver is adjusted so that theattenuation in the seawater is such that the direct wave 18 cannot bedetected. Clearly, the reflected wave 19 would also be attenuated somuch that it also would not be detected, given the greater distance oftravel through the sea water.

The transmitter 13 and receiver 14 are then lowered down together, andtransmissions made at intervals. At a particular depth indicated inbroken lines, the receiver 14′ detected a strong signal following atransmission from the transmitter 13′. This could not be a direct wave,nor a reflected wave, due to the attenuation by the seawater. It wastherefore concluded that the only path for the wave to have taken wasthrough the diaphragm 12. This is shown as a refracted wave 21.

The distance traveled through the seawater is relatively short and whilethe wave traveled some way through the fresh water in the diaphragm 12,the attenuation was considerably less than it would have been throughthe same distance in seawater. Thus, the overall attenuation was lessthan that for the direct wave 18 and the refracted wave 21 was detected.

A more practical example is shown in FIG. 2. The surface of the sea isshown at 31 with the sea 32 extending down to the ocean floor 33. Thereis an overburden 34, an oil-bearing layer 35 and lower layer 36. Thisstructure is known from seismic surveys, but the nature of the layers isnot known. A transmitter is shown schematically at 37 on the ocean floor33 and a receiver similarly at 38. They are spaced apart by an offset39.

The transmitter 37 is in the form of a dipole antenna which is arrangedto transmit an electromagnetic wave polarized in such a way that theradial E component is generally along the line to receiver. This resultsin a direct wave 41 being propagated in the sea water along the surfaceof the overburden and a reflected wave 42 a and 42 b which proceedsthrough the overburden 34, strikes the top surface of the oil-bearinglayer 35 and is reflected. The portions which are received by thereceiver 38 are indicated.

The transmitted wave also results in a refracted wave 43. This iscomposed of a downward portion 43 a which descends through theoverburden 34, a refracted portion 43 b which travels along the layer35, and an upward portion 43 c which travels back up through theoverburden 34. Since the refracted portion 43 b travels much fasterthrough the oil-bearing layer 35 and with far less attenuation, therefracted wave 43 is detected first by the detector 38 and at arelatively high signal level, compared to the direct wave 41 and thereflected wave 42 a, 42 b.

The refracted wave 43 is particularly adapted for determining theboundaries of an oil reservoir e.g., the layer 35, if its depth beneaththe ocean floor 33 is known. This is due to the fact that the downwardportion 43 a of the refracted wave 43 mostly enters the layer 35 at thecritical angle, which is approximately 10° for an oil bearing rock. Atangles of greater than about 15°, total reflection at the surface of thelayer 35 occurs.

Thus, by adopting various positions for the receiver 38, the boundariesof the oil reservoir can be determined, by the absence of an emergingrefracted wave portion 43 c, with accuracy.

This technique also lands itself conveniently to monitoring the changesin a reservoir, over a period of time. The absence of a detectedrefracted wave will mean that the boundary of the oil reservoir hasmoved and the oil content depleted.

In the test layout shown in FIG. 2, the seabed is 1000 m thick, and hasa resistivity of 2 ohm-m. The hydrocarbon layer is about 50-100 m thickand has a resistivity of 50-100 ohm-m.

If the following parameters are then selected: Distance between the Trantenna and the Re antenna=4000 m; Frequency=1.25 Hz; Transmitterantenna and receiver antenna effective lengths L_(T) L_(R)=500 m(antenna physical length 1000 m). Transmitter current 200A.

Then the received signal (direct wave) will be about 5 μV. For f=2.5 Hz,the received voltage becomes 0.5 μV.

When the hydrocarbon layer has sufficiently large width, one can expectthat the refracted wave will be stronger than the direct wave.

1. A method for recovering a volume of hydrocarbons from a reservoirstratum covered by S meters of overburden and having an approximategeometry and location that are known comprising, using a source totransmit a transmitted electromagnetic wave from a location that is atleast S meters distant from the reservoir stratum, with the transmittedelectromagnetic wave having a wavelength of at least 0.1S and no morethan 5S; using a receiver to receive signals created by the transmittedelectromagnetic wave; analyzing the received signals for electromagneticwave refraction signals to detect the presence of a hydrocarbon volumeor water volume in the reservoir stratum; and, after detecting thepresence of a hydrocarbon volume, producing the hydrocarbon volume fromthe reservoir stratum; wherein the source is used to transmit thetransmitted electromagnetic wave as a polarized transmittedelectromagnetic wave, and the source further includes a dipole antennato transmit an E-field component of the polarized transmittedelectromagnetic wave such that the E-field component is approximatelydirected along a line between the antenna and the receiver.
 2. Themethod of claim 1, wherein analyzing the received signals furthercomprises at least one member of the group consisting of using seismicdata generated by a seismic survey, using the results of a mathematicalsimulation model based on a resistivity and a geometry of strata in theoverburden, and using a resistivity of the reservoir stratum andoverburden.
 3. The method of claim 1, wherein the source is used totransmit the transmitted electromagnetic wave having a frequency between0.01 and 20 Hz.
 4. The method of claim 1, wherein the source is used totransmit the transmitted electromagnetic wave as a polarized transmittedelectromagnetic wave.
 5. The method of claim 1, further comprisingtransmitting the transmitted electromagnetic wave when the source iskept stationary, the receiver is kept stationary, or both the source andthe receiver are kept stationary.
 6. The method of claim 1, furthercomprising transmitting the transmitted electromagnetic wave to thereservoir stratum when the reservoir stratum is a submarine reservoirstratum under at least 300 m of seawater.
 7. The method of claim 1,comprising transmitting the transmitted electromagnetic wave from thesource for a length of time between 3 seconds and 60 minutes.
 8. Themethod of claim 1, wherein the hydrocarbon comprises oil or natural gas.9. The method of claim 1, further comprising using a motorized vehiclefor disposing the system in a marine environment.
 10. The method ofclaim 1, wherein the source and the receiver are separated by an offsetdistance that is at least three times S meters.
 11. The method of claim1, further comprising separating the source and the receiver by adistance, L, that is within a range described by the formula 0.5λ≦L≦10λwherein λ is the wavelength of the transmitted electromagnetic wave. 12.The method of claim 1, further comprising suppressing a direct wavegenerated by the transmitted electromagnetic wave.
 13. The method ofclaim 1, further comprising drilling a borehole into the reservoirstratum.
 14. A method for generating data for determining the nature ofa reservoir stratum covered by S meters of overburden and having anapproximate geometry and location that are known, the method comprising:transmitting, from a dipole antenna source, a polarized transmittedelectromagnetic wave with a frequency between 0.01 and 20 Hz at alocation that is at least S meters distant from the reservoir stratum,wherein the transmitted electromagnetic wave has a wavelength of atleast 0.1S and no more than 5S; receiving, at a receiver, signalscreated by the transmitted electromagnetic wave; with the E-fieldcomponent of the polarized transmitted electromagnetic wave beingapproximately directed along a line between the antenna and thereceiver; generating output data from the signals; and analyzing theoutput data for refraction signals to determine the presence ofhydrocarbon or water in the reservoir stratum.
 15. The method of claim14, wherein analyzing the output data further comprises at least onemember of the group consisting of using seismic data generated by aseismic survey, using the results of a mathematical simulation modelbased on a resistivity and a geometry of strata in the overburden, andusing a resistivity of the reservoir stratum and overburden.
 16. Themethod of claim 14, further comprising transmitting the transmittedelectromagnetic wave when the source is kept stationary, the receiver iskept stationary, or both the source and the receiver are keptstationary.
 17. The method of claim 14, further comprising transmittingthe transmitted electromagnetic wave to the reservoir stratum when thereservoir stratum is a submarine reservoir stratum under at least 300 mof seawater.
 18. The method of claim 14, comprising transmitting thetransmitted electromagnetic wave from the source for a length of timebetween 3 seconds and 60 minutes.
 19. The method of claim 14, whereinthe hydrocarbon comprises oil or natural gas.
 20. The method of claim14, further comprising using a motorized vehicle for disposing thesystem in a marine environment.
 21. The method of claim 14, wherein thesource and the receiver are separated by an offset distance that is atleast three times S meters.
 22. The method of claim 14, furthercomprising separating the source and the receiver by a distance, L, thatis within a range described by the formula 0.5λ≦L≦10λ wherein λ is thewavelength of the transmitted electromagnetic wave.
 23. The method ofclaim 14, further comprising suppressing a direct wave generated by thetransmitted electromagnetic wave.
 24. The method of claim 14, furthercomprising drilling a borehole into the reservoir stratum.
 25. Data fordetermining the nature of a reservoir stratum covered by S meters ofoverburden and having an approximate geometry and location that areknown, the data comprising: refraction signals received at a receiverthat are created by a polarized electromagnetic wave transmitted with afrequency between 0.01 and 20 Hz from a location that is at least Smeters distant from the reservoir stratum, wherein the transmittedelectromagnetic wave has a wavelength of at least 0.1S and no more than5S, with the E-field component of the polarized transmittedelectromagnetic wave being approximately directed along a line betweenthe dipole antenna and the receiver, and wherein the refraction signalsare analyzable to determine the presence of hydrocarbon or water in thereservoir stratum.
 26. The data of claim 25, further comprising at leastone member of the group consisting of seismic data generated by aseismic survey, the results of a mathematical simulation model based ona resistivity and a geometry of strata in the overburden, and aresistivity of the reservoir stratum and overburden.
 27. The data ofclaim 25, wherein the refraction signals received at the receivercomprise refraction signals from a submarine reservoir stratum under atleast 300 m of seawater.
 28. The data of claim 25, wherein therefraction signals received at the receiver comprise refraction signalscollected when the transmitted electromagnetic wave is transmitted fromthe source for a length of time between 3 seconds and 60 minutes. 29.The data of claim 25, wherein the hydrocarbon comprises oil or naturalgas.
 30. The data of claim 25, wherein the refraction signals receivedat the receiver comprise refraction signals collected when the sourceand the receiver are separated by an offset distance that is at leastthree times S meters.
 31. The data of claim 25, wherein the refractionsignals received at the receiver comprise refraction signals collectedwhen the source and the receiver are separated by a distance, L, that iswithin a range described by the formula 0.5λ≦L≦10λ wherein λ is thewavelength of the transmitted electromagnetic wave.